pelagic bird vs seafowl

pelagic bird and seafowl both are nouns.

Word	Noun	Adjective	Verb	Adverb
pelagic bird	Yes	No	No	No
seafowl	Yes	No	No	No

As nouns, seafowl is a hypernym of pelagic bird; that is, seafowl is a word with a broader meaning than pelagic bird:

pelagic bird: bird of the open seas
seafowl: a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.

Other hypernyms of pelagic bird include sea bird, seabird.

pelagic bird (noun)	seafowl (noun)
bird of the open seas	a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.